A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
An important step in the lithographic process is the measurement of the target substrate, to ensure high contrast patterning and proper alignment with previously applied patterned features. The level (height) of the surface across a substrate, though it may be in everyday terms extremely flat, typically varies from place to place sufficiently to disturb the quality of the applied pattern if not corrected. In the example of patterning by optical projection, for example, level variations will disturb the focusing of the projected pattern, and hence the contrast between exposed and unexposed parts of the resist. Accordingly, high-performance lithographic apparatuses conventionally include level sensors for ‘mapping’ variations in surface level across the substrate as precisely as possible, prior to applying the pattern. Typically this mapping is performed for the entire substrate in a measurement step, but in principle it may be performed after applying the pattern to a first portion of the substrate.
In one known lithographic apparatus, a level sensor projects a line array of sensing spots onto the substrate at an oblique angle, these being reflected and detected by a photodetector. Each spot includes features such as a grating pattern, which can be compared with a corresponding pattern at the detector side to detect very fine height variations in the target surface. In the known apparatus, the substrate is moved beneath the level sensor in a two-dimensional sweeping pattern, to measure and map level variations across the entire surface. This sweeping movement takes time in the measuring step, which inevitably limits the throughput of the apparatus (wafers per hour). U.S. Pat. No. 5,191,200 (van der Werf et al/Philips) describes examples of level sensing apparatus suitable for use in such an apparatus. The contents of U.S. Pat. No. 5,191,200 are hereby incorporated herein by reference.